The study of the dynamics taking place during power system blackouts is a subject that receives continuous attention in view of its inherent complexity and relevant consequences. Within this context, the paper aims at studying the role of the frequency control reserves (FCRs) on the cascading outages and the relevant short-term dynamics associated with the blackout mechanisms. The relationship between the large and small blackout frequency with respect to the value of FCRs is assessed. More in particular, the main contribution of this paper is to study the influence of the power system interconnections on its pre- and post-blackout behavior. For this investigation, a statistical procedure, based on the Monte Carlo simulation (MCS), is proposed. It performs a blackout risk analysis considering cascading outages as well as generators/loads response to the frequency deviation. The proposed procedure is then applied to the IEEE 118 bus system as an interconnected network characterized by three areas. The distributions and expected values of the blackout size are assessed for the whole system as well as for each area.